[akimball442]

Hi there. Since this is an introductory post, it could get long, feel free to fast forward to the questions at the end.

I'll first introduce myself as an avid 'tube' guy. I'm a guitar player (hopefully that's not taboo around here) and I've been servicing my vintage amps and doing ground up builds for several years now. I have two home stereo systems, and they are all tube powered, and I love them. I'm no stranger to working on amplifier circuits, but RF circuits are something I'm admittedly unfamiliar with, and hopefully, I can learn a thing or two here.

I have a bit more than a rudimentary knowledge of electronics, as I went to school for electrical engineering, but my love of engines and power ultimately got me certified as a diesel technician, and I combine both electrical and mechanical knowledge as someone who troubleshoots modern diesel engines 40+ hours a week.

Now onto radios. I picked up my first Philco about five years ago, because I loved the style, and it was in perfect condition. As an added bonus, with no tape measure handy, I knew it was a perfect fit for a table I have, and it was. That one is a 46-1203, with a record changer and AM radio. It works very well, and other than needing caps when it began to hum about a year ago, it has performed flawlessly.

I have since obtained (as a gift) a 42-380 floor standing unit. It came to me with a near perfect cabinet, and I love the style. Other than the plastic trim bezel, which I understand is still available, it needs little cosmetic reconstruction.

Functionally, it was a mess. The power cord was destroyed. Once I brought it up on the variac, it was obvious that the capacitors were junk. So I replaced the electrolytics, and the cord.

I powered it up, and it worked pretty well, but the audio quality was terrible. I replaced all of the various coupling caps with very decent caps. Probably a waste on a low-fi radio, but it's what I had. The audio quality improved dramatically. I'm not impressed with the treble control, but that's another thread.

I cleaned all the controls, replaced the lamps, cleaned the dial face, and put it back together.

It works, on all bands. But there's two issues, one major, one minor.

The minor issue is that this is obviously a local unit, because the presets are all local stations. The reception on them is faint compared to manual tuning of the same stations, however.

The major issue is that after the unit is on for a short time, distant stations fade. Local stations come in strong for as long as I want to listen to it. But an example is that WSM Nashville comes in strong on my car radio in the early morning (I'm in Central NY). When I turn the unit on, initially, the station is very strong with little or no noise. After a short time (2-3 minutes) the station begins fading, until it is unreadable by 10 minutes.

What could be causing this problem? Every single cap in the unit has been replaced, so it's unlikely a cap. I'm thinking a resistor could be getting hot and going out of range, but I'm not sure where to check.

Any assistance would be appreciated, as I would enjoy listening to the radio while I get ready for work in the morning.

Thanks a lot!

Aaron

[Radioroslyn]

Does this fading happen on other radios also?? If so I would suspect that it is just the distant radio signals being absorbed by the atmosphere.
Terry

[akimball442]

No, the fading only occurs on this radio. The other Philco I have does not pick up as well, but the reception is reasonably steady. I surmise this one gets (at least initially) better reception is because it has a superior antenna system.

The Nashville station comes in reasonably well starting around midnight, and reception is good until a little after sunrise on most days. Storms in between can cause varying results, obviously.

It does not matter when I turn the radio on, within 10 minutes, the reception on weak stations fades to a nearly unreadable signal. Before the set warms up, weak and distant stations come in very well. Stronger signals may also fade, but they come in well enough that you might not notice. For example, there is a program that I listen to during the day on Sunday, on a local station, and I can listen to it with no problems. The issue seems to only be present with weak or distant stations.

Thanks for the reply!

Aaron

[Conelrad]

Sounds like you may have a tube going soft, or a bypass/coupling cap (maybe hidden in an IF can) dropping the ball.

Both things happen sometimes when energized for a bit.

Dennis

[akimball442]

So there are caps INSIDE the IF cans? If there were a bad tube, is there one in particular I should suspect?

[Arran]

I would check all of the resistors, if any are more then 10% out change them. This may also explain the fidelity problems, a bias resistor is going wonky messing with the electrical characteristics of the tube.
Regards
Arran

[TA Forbes]

Also may want to check oscillator circuit for a leaky mica cap, may cause drift.